Add admin command to display last snapshot information

Basic systems to backup ZooKeeper data will maintain snapshot files of the data tree. In order to understand the health of these systems, they need a way to determine how in or out of date their files are to the current state in the ensemble.

Add an admin command that exposes the zxid and timestamp of the last saved/restored snapshot of the server. This will let such a backup system know when it can update and when it is stale.